Unit 4 · Earth's Restless Surface · Week 2
Why do the continents fit like puzzle pieces?
If you look at a world map, South America's east coast and Africa's west coast look like they could snap together. Scientists found the same fossils and rock bands on both sides of the ocean — separated by thousands of kilometres of water. What is going on?
Start here
How we work this lesson (our expectations)
- Let the data lead — what the evidence shows, not what "makes sense"
- Patterns are clues — we look for them before we explain them
- Good science arguments name the evidence, not just the conclusion
- Every support panel is yours — open any, any time
30-second check-in self-awareness + social awareness
Before we start, think back to last week's exit ticket: you predicted what might be moving the continents. Today we find out if you were right. Notice how that feels — being right is satisfying, but being slightly wrong and then understanding why is even better for learning. Either way, you are ready.
Do-Now Spiral retrieval
Three questions — one spiraling Week 1, one spiraling Unit 1, one new. Pull from memory first.
- Week 1 spiral Name Earth's four layers from outside to center. Which layer is liquid? Which one does the crust sit on top of?
- Unit 1 spiral Heat moves from hot objects to cooler objects. In the mantle, the rock closer to the hot core is hotter than the rock closer to the cool crust. What do you predict happens to that hotter rock? (Think about what hot air does in a room — it rises.)
- New: If you found the exact same species of ancient fern fossil on two continents separated by 7,000 km of ocean, what are two possible explanations for how the same fern ended up on both continents?
The phenomenon
In the early 1900s, Alfred Wegener noticed something strange: the eastern coastline of South America and the western coastline of Africa looked as if they could fit together like two pieces of a broken puzzle. He didn't just notice the shape — he also found that the same species of fossil plants and animals appeared on both continents, separated by thousands of kilometres of open ocean. The same bands of rock type matched up on both sides. Wegener proposed that the continents had once been joined — and had slowly moved apart. Most scientists at the time rejected the idea because they could not imagine what force could move an entire continent. Later, when the discovery of seafloor spreading showed that new rock was forming at ocean ridges and pushing the old rock outward, the mechanism was finally understood: the plates carrying the continents were being moved by convection currents in the mantle below. Our question: what is the engine that moves the continents — and how is it powered?
South America and Africa look like puzzle pieces. Scientists also found the same fossil animals and the same rock bands on both continents. Wegener said the continents used to be joined and then moved apart. The force that moves them is heat from inside the Earth. Hot mantle rock rises, spreads sideways, and drags the crust along — this is called convection. What moves the continents, and what powers it?
Notice & Wonder. Write one pattern you notice in the diagram. Write one thing you wonder about why that pattern exists.
Build it Three lines of evidence + one engine
We build the case for plate tectonics from evidence first, then we reveal the engine that makes it work.
Step 1 — Three kinds of evidence for moving continents
Scientists accept plate tectonics because multiple independent lines of evidence all point to the same conclusion. (1) Fitting coastlines: the shapes of South America and Africa align as if cut from the same piece of rock. (2) Matching fossils: the same extinct land-animal species — including Mesosaurus, a freshwater reptile that could not swim the Atlantic — appear on both continents. If the continents were always separated, the same species could not have lived on both sides of an ocean. (3) Matching rock bands: ancient mountain chains end at the African coast and pick up again in South America at exactly the right age and rock type — as if they were once the same mountain range, split and separated by the opening ocean. Together these three lines of evidence make the case that the continents were once joined.
Three kinds of evidence show the continents used to be joined: (1) their shapes fit like puzzle pieces; (2) the same fossils appear on both sides — including a freshwater animal that could not cross an ocean; (3) the same rock bands line up on both coastlines. All three match — that is too many patterns to be a coincidence.
Step 2 — Mantle convection: the engine
The engine that moves the plates is mantle convection. Hot rock near the core is less dense than cooler rock above it, so it rises slowly — just like hot air rises in a room (Unit 1/Unit 3 throughline). As the hot rock rises to near the surface, it cools, becomes denser, and sinks back down. This circular movement — rise, spread, cool, sink — is a convection current. Where the rising hot material reaches the top, it spreads sideways and drags the plates above it in opposite directions. At mid-ocean ridges, the spreading creates new seafloor. This is seafloor spreading, and it is the direct mechanism that pushed South America away from Africa over millions of years. The energy source is exactly what Week 1 established: heat flowing outward from Earth's hot interior.
Hot mantle rock rises from near the core (it is less dense, like hot air that floats up). At the top it spreads sideways and cools, then sinks. This loop — rise, spread, sink — is a convection current. The spreading pulls the plates apart at ocean ridges. New seafloor is made. Old continents drift apart. The power source: heat from inside the Earth.
Worked example — building the argument from evidence (watch the reasoning steps)
Evidence 1 (fossils): Same Mesosaurus fossil on two continents. It was a freshwater animal — it could not swim 7,000 km of saltwater. Possible explanations: (a) it evolved twice independently (very unlikely), (b) a land bridge connected them (no evidence), (c) the continents were once joined.
Evidence 2 (rock bands): Same ancient mountain chain, same rock type and age, on both sides of the Atlantic. Very unlikely if the continents were always separated. Most likely: they were once one mountain range.
Evidence 3 (seafloor age): Rock at the mid-ocean ridge is youngest; rock farther from the ridge is progressively older — exactly what seafloor spreading predicts. New rock forms at the ridge and the old rock pushes outward.
Conclusion: Multiple independent lines of evidence all converge on the same explanation — continents move, powered by convection in the mantle. This is how science works: one line of evidence suggests, multiple lines confirm.
Show me how convection works as a diagram Tier 2 · visual
Draw a rectangle representing the mantle. Draw arrows: hot rock near the bottom rising (arrow up, label "hot, less dense"), spreading at the top (arrow sideways, label "spreading pulls plates"), cooling and sinking on the sides (arrow down, label "cool, denser, sinks"). Label the heat source at the bottom (Earth's core). Now label where the crust is (on top) and where a mid-ocean ridge would be (directly above the rising column). This is one convection cell — there are many across Earth's mantle.
Your turn Make the argument for moving continents
Use a Claim–Evidence–Reasoning format. Choose your response mode — all are equally valid.
The task: A classmate says "continents can't move — land doesn't float on water." Use at least two lines of evidence and the convection explanation to convince them. Make a claim, give evidence, and explain your reasoning.
I want sentence starters Tier 2 · frames
Claim: The continents do move because ________.
Evidence 1: Scientists found ________ on both South America and Africa. This matters because ________.
Evidence 2: The rock bands on both continents ________. This shows ________.
Reasoning: The engine that moves the continents is ________. It works because hot rock ________, spreads sideways, then ________. This is the same idea as ________ rising in a room (Unit 1).
Word bank: convection · mantle · hot · dense · rises · cools · sinks · plates · fossils · evidence · spreading · ridge · continents · energy · heat
Let's make it concrete first Tier 3 · re-teach
Analogy: Put a pot of tomato soup on the stove. Watch near the bottom: hot soup rises in columns in the center, spreads out across the top, cools, and sinks at the sides. If you put crackers on the surface, the spreading soup pushes them apart. That is exactly what the mantle does with the crustal plates — the crackers are the continents. The stove burner = Earth's hot interior. The soup = the mantle. The crackers = the crustal plates.
Fill in: The continents moved apart because hot mantle rock ____ (direction) near the core, spread ____, and dragged the plates ____. This is called ____. The energy came from ____.
Say your answer out loud before writing. One sentence per piece of evidence is enough.
Push my thinking Extension
Wegener was largely dismissed during his lifetime (he died in 1930) because he could not explain the mechanism. The mechanism (seafloor spreading and convection) was not confirmed until the 1950s–60s when geologists mapped the ocean floor. Research the story of Marie Tharp, who created the first detailed map of the mid-ocean ridge system in the 1950s. How did her evidence change the scientific community's view of Wegener's hypothesis? What does this story tell us about how science changes its mind — and how long it can take? Connect to this unit's CCC of Patterns: what patterns did Tharp find, and what did those patterns mean?
Check yourself Did it click?
- name at least two independent lines of evidence for plate tectonics and explain why each supports moving continents;
- describe mantle convection (hot rises, spreads, cools, sinks) and connect it to the same heat-transfer idea from Unit 1;
- explain that plates move because convection in the mantle drags them — not because they float on water.
Which piece of evidence most directly disproves the idea that the same fossil species on two continents got there by swimming across the ocean?
Re-teach: land can't float on water — so what does it float on? Tier 3
The continents don't float on water — they rest on the mantle, which is solid rock that behaves like very slow putty over millions of years. The plates are like giant slabs floating on that slow-moving mantle. The mantle moves because of convection: hot rock near the core is less dense (like hot air) so it rises, spreads sideways at the top, drags the plates along, then cools, becomes denser, and sinks. The energy comes from Earth's internal heat (Week 1). So the mechanism is convection, not floating on water. The fossil and rock-band evidence tells us the plates have in fact moved — the convection current is how.
Reflect, then show what you know
Reflection metacognition + responsible decision-making
Wegener had the right idea but was ignored for decades because he couldn't explain the mechanism. Think of a time you had a good idea that someone dismissed before you could explain it. How did that feel? What would have helped? In science, evidence is the key — not just the idea.
Exit ticket — answer on paper or in your notebook.
- New Describe mantle convection in your own words. Use the words hot, rises, cools, and sinks.
- New Name two lines of evidence that support the idea that South America and Africa were once joined. For each, explain why that evidence is convincing.
- Spiral — Week 1 Where does the heat that drives mantle convection come from? Connect to what you learned about Earth's interior.
- Spiral — Unit 1 In Unit 1 we learned that hot things rise and cold things sink in fluids — this is convection. Name one other place in nature where you have seen this pattern at work. (Hint: Unit 3 weather?)
- Transfer Next week we will look at what happens when plates collide instead of spreading apart. Predict: what do you think happens at the boundary where two plates crash into each other? What would the evidence look like? (Best guess — we answer it next week.)
Educator panel — facilitation, accommodations, answer key
At a glance
Standard: NGSS MS-ESS2-3 — Analyze and interpret data on the distribution of fossils and rocks, continental shapes, and seafloor structures to provide evidence of the past plate motions. 3D: SEP Analyzing & Interpreting Data + Constructing Explanations · DCI ESS2.B Plate Tectonics · CCC Patterns + Energy & Matter + Cause & Effect. Spiral connections: Unit 1 (heat transfer/convection) and Unit 3 (atmospheric convection) — same physical principle, new scale.
Materials (optional, fully digital-capable): student notebook; embedded SVG; optional world map to physically trace the coastlines.
Facilitation arc (Rosenshine)
- Open by sharing student predictions from last week's exit ticket — affirm both right and slightly-off predictions (3 min).
- Do-Now retrieval (3 min).
- Build Steps 1–2 with the worked example; cold-call on each evidence type (12 min).
- Differentiated CER — circulate; ask "which line of evidence is most convincing to you?" (15 min).
- Self-check + re-teach branch (5 min).
- Brain break, reflection, exit ticket (10 min).
IEP / 504 accommodation cards
Common accommodations mapped to this lesson
| Accommodation | How it's already built in / what to do |
|---|---|
| Text read aloud | Every passage has a Listen button (TTS); simplified toggle shortens it. |
| Reduced reading load | "Reading: Simplified" swaps in plain-language text. |
| Chunking / extended time | Small steps with checks; progress saves automatically. |
| Sentence frames / word bank | Tier-2 panel provides both for the CER task. |
| Concrete manipulatives / models | Tier-3 "tomato soup" convection analogy; offer a world map to trace physically. |
| Alternative response mode | Students may draw, speak/record, or write the CER — all accepted. |
| Behavior / focus (504, ADHD) | Named expectations (PBIS), 30-sec check-in, built-in movement brain break. |
| Multilingual / EL | Simplified text, SVG visual, word bank, home-language reflection allowed. |
Exit ticket answer key
- Hot mantle rock near the core is less dense, so it rises toward the crust. There it spreads sideways and cools. The cooler, denser rock then sinks back down. This circular loop — rise, spread, cool, sink — is mantle convection.
- Accept any two: (a) Fitting coastlines — the shapes of South America and Africa match as if cut from the same rock; (b) Matching fossils — e.g., Mesosaurus appears on both continents and was a freshwater animal that could not have crossed the ocean; (c) Matching rock bands — the same ancient mountain chains and rock ages align on both coasts.
- The heat comes from Earth's hot interior — radioactive decay of elements in the rock and leftover heat from Earth's formation (Week 1).
- Accept any reasonable convection example: warm air rising near a heater and cold air sinking; ocean currents (warm water rises at the equator, cold sinks at the poles); storm cells (warm moist air rises); even lava lamps.
- Predictions will vary — accept any that include: the plates could crumple, one could go under the other, mountains could form, the ground could shake. All these are addressed in Week 3.
Why it's designed this way
Multiple lines of evidence (NRC 2012, SEP Analyzing Data): rather than stating "plates move," students are walked through three distinct evidence types so they can argue from evidence. The self-check addresses the most common misconception (land floats on water) directly, routing to a concrete re-teach. Spiral connections (Unit 1 and 3 convection) strengthen the year-long throughline — same concept at multiple scales. The transfer item previews W3's boundary interactions so that lesson opens with student predictions as the launch data (P6, interleaving). The Wegener story is included for equity: it models persistence in the face of dismissal, and names the history of science as a social, not just intellectual, process (P11).